The length of windmill arms increases in line with the nominal power of the windmill. For windmills with a nominal power of 3-4 MW and above, arms with lengths of 60 meters or more are used. Such mill arms are difficult to transport on account of this length. It is therefore desirable for it to be possible to transport such arms in sections. However, it is a problem to assemble such sections in a simple way when reproducibility and safety are to be ensured in the assembly, which is subjected to great dynamic stresses and therefore has to be capable of being maintained.
It is known that windmill arms can be manufactured in sections, which are subsequently assembled to form a whole.
In H. Hald et al: “The DFVLR—25 m—Wind Energy Converter”, European Wind Energy Conference, 1984, a method is described in which two arm sections in a butt joint are held together with longitudinal bolts which are fastened to nuts fitted in transverse holes in the arm shells. The disadvantage of this method is that there is limited space for the requisite tightening tool at the bolt which is tightened when assembly takes place.
The patent EP 1 244 873 describes a method in which two arm sections in a butt joint are held together with longitudinal metal straps which are prestressed by rotating eccentric bolts. The disadvantage of this method is that the prestressing tolerance is limited and that it is difficult to ensure correct prestressing on account of the great friction in the strap joint.
WO 04015265 describes a method in which two arm sections in a butt joint are held together with a glued-on overlapping plate shaped like an arm profile. The disadvantage of this method is that it involves making a glued joint under field conditions with the associated risks of it not being possible, because of the surrounding conditions, for the joint to be made to the same quality as if it were made under controlled conditions. Furthermore, the arm cannot be separated again, for example in the event of damage which is limited to the outer arm alone, and, in the event of being transported away, the arm joint has to be cut through destructively.